1. Field of the Invention
The invention relates to a sheet feeding apparatus and an image forming apparatus and an image reading apparatus provided with the same, and particularly to a construction for separating and transporting sheets one by one.
2. Description of Related Art
In recent years, an image reading apparatus and image forming apparatuses such as a printer, a copying machine, a facsimile apparatus and a compound machine having a plurality of functions are provided with a sheet feeding apparatus for separating and feeding sheets such as originals or recording sheets one by one to an image reading portion or an image forming portion. As such a sheet feeding apparatus, use is widely made of one adopting a retard separation mechanism which can separate sheets highly reliably from a low speed to a high speed.
This retard separation mechanism is designed to separate and feed sheets one by one by a feed roller rotated in a sheet feed direction and a separation roller (retard roller) urged against the feed roller with predetermined pressure and rotated in a direction opposite to the sheet feed direction, in other words, a direction for returning the sheets.
In the drive transmitting path of this separation roller, there is provided a torque limiter for creating slip at a predetermined torque value or greater so that in the absence of a sheet or in a state in which a sheet is being transported between the feed roller and the separation roller, a load comprising a frictional force with the feed roller or the sheet may reach the predetermined torque value or greater of the torque limiter and the torque limiter may slip and the drive may come to be not transmitted, whereby the separation roller may be driven to rotate (be taken to rotate) in the sheet transport direction, whereby a sheet become capable of being reliably transported without being damaged.
Also, in a state in which a plurality of sheets superposed one upon another have come into the pressure contact portion (hereinafter referred to as the nip portion) between the feed roller and the separation roller, the separation roller is designed to be reversely rotated without being driven to rotate, whereby a sheet in contact with the feed roller may be transported in the feed direction by the slip between the sheets, and a sheet in contact with the separation roller may be returned to a sheet supporting portion by the reverse rotation of the separation roller.
To satisfy the transport and separation condition in the above-described construction, it is necessary to satisfy the following expressions:F1>F3>F4>F2  (1)F5>F4,  (2)where                F1=μ1×N: the force with which the feed roller advances the sheet        F2=μ2×N: the force with which the sheets take each other        F3=μ3×N: the force with which the separation roller stops the sheet        F4=T÷r: the force with which the separation roller is taken to rotate        F5=μ4×N: the force with which the feed roller rotates the separation roller        μ1: the coefficient of friction between the feed roller and the sheet        μ2: the coefficient of friction between the sheets        μ3: the coefficient of friction between the separation roller and the sheet        μ4: the coefficient of friction between the feed roller and the separation roller        N: the pressure force of the separation roller        r: the radius of the separation roller        T: idle rotation torque of the torque limiter.        
Now, there has heretofore been a retard separation mechanism as shown in FIGS. 7A and 7B of the accompanying drawings wherein in order to secure more reliable separating performance, a separation roller 52 is constituted by an elastic member and near the separation roller 52, there is provided a transport restriction guide 54 adapted to be relatively protruded toward a feed roller 51 by the deformation of the elastic member.
This transport restriction guide 54 is formed so that as it approaches the nip portion between the feed roller 51 and the separation roller 52, the interval between it and the feed roller 51 may become narrower. Also, this transport restriction guide 54 is formed by a flexible member and the surface thereof adjacent to the feed roller is formed with a rugged shape abutting against the feed roller 51.
By thus constituting the separation roller 52 by an elastic member, and providing the transport restriction guide 54 adapted to be relatively protruded toward the feed roller 51, even if a sheet stack comprising a number of sheets superposed one upon another comes into the nip portion, a portion of the sheet stack having come in which is adjacent to the separation roller can be received by the rugged shape of the transport restriction guide 54 without hindering the operation of the separation roller, as shown in FIG. 7B, whereby only a few sheets separable can be made to come into the roller nip portion.
Also, even from a state in which the leading edge of a plurality of sheets having come in is caught by the rugged shape of the transport restriction guide 54, as the number of sheets is decreased by the separating operation, the separation roller 52 comprising an elastic member pushes out the sheets and therefore, the bad feeding of the sheets can be avoided.
As another sheet separation mechanism, there is a separation pad mechanism provided with a feed roller rotated in a sheet feed direction, and a frictional pad urged against the feed roller with predetermined pressure, and separating and feeding sheets one by one.
However, in a sheet feeding apparatus adopting such a retard roller mechanism, in the case of a vertical type sheet feeding mechanism designed such that for example, a sheet is inclined and is made to come into the nip with the aid of the gravity of the sheet, a sheet having a high coefficient of friction, coupled with the gravity of the sheet S, cannot sometimes be received by the transport restriction guide 54 and a plurality of sheets may come into the nip like a sheet.
When such a situation happens, F2 (the force with which the sheets take each other) mentioned above becomes high and exceeds F4 (the force with which the separation roller is taken to rotate) and the separation roller 52 is taken to rotate by the sheet stack, and the sheet stack having come into the nip is intactly double-fed.